Hypothesis: A cheap rocket motor igniter can be made out of steel wool and black powder.
We tried out the concept by sending an electrical current through some steel wool and got it to ignite at 3 Volt and 5 Ampère.
Power required = 3 Volt * 5 Ampère = 15 Watt
Readers note: steel wool requires quite some energy to get it to glow red hot and ignite. But 15 Watts is easily reached by using a powerful battery such as an 28 Volts battery of an electrical hand-held drill.
Here's the video of steel wool igniting at around 15 Watts:
Then we arranged the steel wool on some paper tape:
Poured black powder on top and simply wrapped it up.
We made 3 versions:
test #1: a black powder igniter with paper tape, pictured above
test #2: a black powder igniter with a plastic straw
test #3: a black powder igniter with a post-it, rolled up and taped together
Test #3 contained a bit too much black powder than strictly necessary, check out the video. It will be discussed in the"Safety Improvements" section below!
Price
Small amount (2 g) of steel wool: 0.2 euro
Small amount (10 g) of black powder: 0.6 euro
Copper wire (10 cm) to connect to the steel wool: 0.2 euro
Total price per igniter: around 1 euro
Conclusions
The hypothesis has been validated; a cheap rocket motor igniter can be made out of steel wool and black powder. The igniter has a power requirement of around 15 Watt.
Safety improvements
To stay focused on safety, we'll conclude every session by trying to find at least one safety improvement to be implemented.
Next time we'll be sure to fixate the camera and leave it unmanned. That will give us a better view of the reaction from close by and will keep us safe at the same time.
This simple, cheap home-made powder mixer can be used to mix relatively large quantities of powder, such as blackpowder, automatically.
Since a picture says more than a 1000 words, let's start with 160 pictures in rapid succession:
Above: The powder mixer in action. This video says more than 160 x 1000 words.
Note that the final model looks slightly different; in the end we moved one of the rolls to the opposite side of the box so that the mixing container is spinning between the powered and the non-powered roll. This reduces the friction on the container so that it spins around more easily.
Shopping list
1 hand-held drill (battery powered or otherwise is fine) to provide rotation
1 plastic box (cheap at IKEA or simular) to support the axles
2 cardboard cylinders
2 axles (we used metal bars but wood should also work)
glue to attach the cylinders to the axles
Mixing container
The mixing happens inside a container. We used a metal container with a plastic lid, which is not ideal for pyrotechnic powders - see the "Safety Improvements" section below.
To ensure proper mixing, we glued a wooden stick to the inside of the mixing container. That way, the powder gets scooped up and dropped down continuously, similar to a concrete mixer.
We also threw in a few plastic cubes, again to have better mixing by creating some "chaos" or randomness in the mixing process.
On the picture below, the wooden stick is barely visible because of the blackness of the powder:
The powder mix container filled with black powder.
The results
This blackpowder is of the best quality that we have made with any dry mixing process so far.
The only superior quality that we obtained in the past was with a liquid mixing processes, such as the process that involves boiling water, a blender and isopropyl alcohol, but that is more cumbersome and the quality we have here will suffice for our igniters.
In hindsight, mixing black powder in a metal container with a plastic lid is not ideal for safety. Should the black powder ignite for whatever reason, and should the plastic lid fail to release, then the pressure would build up inside the container and it could potentially blow up.
In general, it is recommended never to place pyrotechnical substances inside metal or glass containers because they could shatter when the substance ignites. Also, make sure that the container has a lid that easily comes off to ensure no pressure can build up inside.
We resumed our GALCIT solid rocket propellant preparation that was started a few days ago.
High-level plan:
Melting the mixture of bitumen and paraffin oil that we made last time
Mixing in 2614.4 grams of potassium perchlorate (KClO4)
Casting the GALCIT into some cardboard test engine tubes and into a small ceramic bowl
Lighting a small amount of GALCIT at atmospheric pressure in the ceramic bowl
Step 1. Melting the mixture of bitumen and paraffin oil
Melting the mixture of bitumen and paraffin oil took around 1 hour at 300 degrees Celcius. This is quite slow because the contact surface area between the melting beaker and the temperature controlled hotplate is quite small. To optimize this, we'll use a crucible with a larger base next time.
For safety, a gas mask and protective gloves were worn during the mixing. The gloves are great, they protect from heat and fluids so they are ideal for bitumen. Without them, this would have been a lot more difficult.
Once the mixture had completely melted, it was quite runny, quite comparable to molten chocolate.
Step 2. Mixing in potassium perchlorate
Mixing in the oxidizer was done in open air for safety reasons, again with protective gear.
The large volume of propellant in comparison with the small surface contact area with the hot plate meant it was difficult to keep the GALCIT hot enough.
And since the oxidizer has quite a high heat capacity (111.35 J/mol·K), every time a quantity of the oxidizer was added, the mixture would cool down and stiffen up. Also the volume would increase, so the unfavorable volume/contact area would worsen at every increment.
In the end, we divided the 2l mixture over two containers and continued with 1l of volume.
That resolved the heating issue and allowed us to finish up our first liter of GALCIT.
The final GALCIT mixture is quite viscose (similar to wet clay) and very sticky.
Step 3. Casting the GALCIT
The viscosity of GALCIT was too high to allow pouring it directly into our cardboard test engine tubes but it was easy enough to scoop it out, smear it into the tubes and pack it together with a stick.
We filled up 3 cardboard test engine tubes that had a clay stopper on the bottom, those still need to be reinforced, fitted with an ignition and nozzled up.
To avoid pockets of air in the propellant, which would dramatically increase the surface area, we just tried to pack it tightly. In the future, we might need a better way to ensure no bubbles get trapped inside to ensure a continuous burn.
Step 4. GALCIT burn test
Since we had a bit of GALCIT left, we decided to do a small test to see whether we could get it to ignite at atmospheric pressure with 10 grams of (relatively poor quality) black powder that we made on the side.
We placed around 14cl (around 300g) into a small ceramic bowl and poured the black powder on top.
This batch of black powder did not easily ignite when placing a burning cigarette into it, although the previous batch did. But that wasn't unexpected, considering the poor quality of the black powder. Luckily, we had a solid plan B for igniting the black powder and the GALCIT, using a simple hand-held propane blow torch.
The blow torch did the trick. We were surprised of how well GALCIT burns at 1 bar, considering it burns optimally at 70-140 bar.
Just take a look at the video, at 1m 10s...
Timeline of the test burn:
1:10 start of blackpowder ignition
1:20 start of GALCIT ignition
2:05 fade out of GALCIT burn (smaller diameter and surface area at bottom of ceramic cup)
2:13 end of GALCIT burn
Total burn time: 45 seconds
A total burn time of 45 seconds is huge for such a small amount of GALCIT. It means the burn rate was very slow, about 0.27 mm/s. Consider the excessively high burn rates (~ 36.5 mm/sec) that have been reported and large burning surface area, estimated at around 40 cm3.
Yet this is to be expected, due to the low atmospheric pressure at which the burn occurred.
GALCIT has a very high pressure exponent so the burn rate increases dramatically with the pressure.
So the time has come to test the propellant at a higher chamber pressure!
